Polyolefins are general purpose plastics having superior chemical resistance and formability (processability), but are not suitable for preparing articles which require the high heat-resistance and superior physical properties due to their inferior heat-resistance and physical properties. To make up for the weak points of polyolefin, polyolefin has been conventionally reinforced with various organic or inorganic materials. For example, the mechanical strength and heat resistance of polyolefin can be improved by mixing polyolefin and glass fibers with a kneader such as an extruder. However, most of the glass fibers are damaged by the screw of the extruder during the mixing step, and the damaged short glass fibers are mixed with polyolefin. Therefore, the glass fibers are not satisfactory for improving the mechanical strength of polyolefin, and there is other disadvantage that the stirring of the mixture of polyolefin and the glass fibers is difficult when the amount of the glass fibers is more than 30 weight %.
In the automobile industry and electronics industry, there is an increasing demand for polyolefin having high impact strength, but the short fiber reinforced polyolefin cannot meet such demands. In order to prepare a long-fiber reinforced polyolefin, filament fibers were disentangled from a bundle of non-polyolefin filament fibers, or from a mixed bundle of polyolefin filament fibers and non-polyolefin filament fibers, and modified or normal polyolefin was impregnated, coated, and/or adhered to the disentangled filament fibers. For example, the conventional long-fiber reinforced polyolefin was prepared by continuously coating minute filament fibers with polyolefin, and the coating step was carried out (1) by simply heating filament fibers while adhering the powdered suspension of modified or normal polyolefin to the filament fibers, (2) by impregnating or contacting filament fibers to a heat melted polyolefin, (3) by adhering modified or normal polyolefin powder to the charged filament fibers, and then heating the polyolefin powder, or (4) by impregnating filament fibers to a solvent-dissolved modified or normal polyolefin, and then removing the solvent. The molded articles produced with the conventional long-fiber reinforced polyolefin have superior mechanical strength, and particularly, have impact strength which is more than three times higher than that of normal polyolefin. The filament fibers are oriented in the same direction in polyolefin, and can be densely filled (90 weight % at maximum) in polyolefin.
The fiber reinforced polyolefin of high density can be used as a master batch for producing the molded articles. As the method for preparing the molded articles by using the fiber reinforced polyolefin as a master batch, the following methods are known in the prior arts. U.S. Pat. No. 5,213,889 disclosed a method of producing molded articles with more than 30 weight % of the long-fiber reinforced thermoplastics which includes more than 50 weight % of the filament fibers in the thermoplastics. Japanese Laid-open patent No. H1-241406 disclosed a molding method which mixes in dry state a long-fiber reinforced thermoplastic and a thermoplastic without the long-fibers. Japanese Laid-open patent No. H3-114705 disclosed a method of preparing long-fiber reinforced polyolefin by using polyolefin having a melt flow rate of 20 to 60 g/10 min. Japanese Laid-open patent No. S59-20339 disclosed a method of heating the mixture of glass filament fiber and modified polyolefin fiber. Korean Patent No. 93-336304 disclosed a method of preparing long-fiber reinforced polyolefin having the glass fiber content of 60 to 90 weight % by using polyolefin of a melt flow rate of 70 to 300 g/10 min, wherein the long-fiber reinforced polyolefin is used as a master batch for preparing the molded articles.
Meanwhile, in order to give coloring to the fiber reinforced polyolefin, a colored polyolefin pellet including condensed dye or pigment is conventionally used. The length of the conventional colored pellets is less than 3 mm, the amount of dye or pigment in the pellet is more than 30 weight %, and the pellets are used in the amount of less than 3 weight %. However, it seems that the drawbacks, which occur when using such small amount of the colored polyolefin pellets which include concentrated dye or pigment, were not seriously considered by the skilled person in the arts. For example, the physical properties of the molded article are seriously deteriorated when the colored polyolefin pellets are mixed with the long-fiber reinforced polyolefin, but such drawback has not been expressly pointed out by the skilled person in the arts. The present inventor found that even small amount of the colored polyolefin pellets would seriously deteriorate the physical properties of the long-fiber reinforced polyolefin, and it seems that the deterioration of mechanical properties of the final molded article was caused by the dye or pigment which deteriorates the compatibility between the long-fibers and the plastic resins at their contacting surfaces. To prevent the deterioration of the mechanical properties due to the dye or pigment, Korean Laid-open Patent No. 10-2000-0063675 disclosed a method of adding one or more polyamide into the colored and long-fiber reinforced articles. However, polyamide is much more expensive than polyolefin, which results in the increase of the production cost of the final molded article. In addition, there is other problem that polyamide is difficult to be blended with polyolefin due to its low compatibility with polyolefin.